The use of a touch input device disposed over the viewing surface of a computer display to provide a "user friendly" means for the control of a data processing system is well known in the art. These devices are designed to allow an unsophisticated user to perform desired tasks on a computer system without extensive training. Human factor studies have shown that an input device which allows the user to input data directly on the computer display generally known in the art as a touch input device, achieves greatest immediacy and accuracy between man and machine.
One of the first input devices for use at the display surface was the light pen. The light pen is an optical detector in a hand held stylus, which is placed against the face of the cathode ray tube in the computer display. The light pen detects the dot of light which is the scanning raster of the CRT. By determining the coordinates of the raster as it is sensed by the light pen, the computer system locates the light pen on the display.
One touch input device uses a frame which fits around the display screen having a number of infrared or visible light transmitters and receptors arranged in parallel horizontal and vertical directions. When the user's finger blocks the light beams, the horizontal and vertical receptors note the absence of the signals, thereby locating the position of the action desired by the user.
Another class of known touch input devices use transparent overlays placed over the display screen. These touch overlays use a variety of means of detecting the presence and location of either a stylus or finger on or near the display surface. One overlay type is a mechanical deformation membrane, a transparent overlay which consists of two transparent conductor planes disposed on a flexible surface. When a selection is made, the user mechanically displaces one of the conductor planes to touch the other by a finger or stylus touch, thereby bringing the conductors into electrical contact. Appropriate electronics and software translate the electrical signals generated by the finger or stylus touch to the position on the display surface. Other touch overlays use capacitive or resistive means in the transparent overlay and associated electronics, to detect the input of a user by finger touch. This type of input device can detect the location of a finger touch by the change in capacitance or impedance of the overlay. Yet other touch overlays use a stylus for user input, either to change the capacitance or impedance at the touched location on the touch screen or to return the electromagnetic or electrostatic signals generated by the overlay back to a microprocessor to determine the stylus position.
Most of the prior art touch devices described above accept input either from a stylus or by a finger touch, but not both. If they can sense the presence of either a finger or a stylus, they generally do not distinguish between the two. In the current graphical user interfaces developed to aid man-machine interaction, there are many items, such as menu selections, icons or windows, which a user can most easily select by a finger. In other advanced software applications which perform freehand drawing, gesture recognition or handwriting capture, a stylus is more effective because of its greater precision. Thus, it would be convenient to utilize a touch input system which allows both stylus and finger touch detection.
One such system is described in U.S. Pat. No. 4,686,332, to E. Greanias, et al., entitled "Combined Finger Touch And Stylus Detection System For Use On The Viewing Surface Of A Visual Display Device" filed Jun. 26, 1986, which is hereby incorporated by reference. This touch input system allows for both finger touch and stylus detection. The system includes a touch overlay sensor which comprises an array of horizontal and vertical transparent conductors arranged on the viewing surface of the visual display device. The conductor array emits electromagnetic or electrostatic signals at 40 kHz into the region above the display surface under the control of a microprocessor. A stylus "antenna" is connected to an input of the detector control system and senses the signals emitted by the array. The signal amplitude sensed by the stylus is related to the position of the stylus on and above the display and the particular conductors being energized. Radiative signal measuring means coupled to the stylus measures the electromagnetic or electrostatic signal received by the stylus.
The finger touch sensing system in U.S. Pat. No. 4,686,332 is a capacitance measurement means which measures the capacitance of selected conductors and determines where and when a finger touch occurs. The system also includes a means for either connecting the output of radiation source or the capacitance measurement means to selected patterns of horizontal and vertical conductors in the array for stylus or finger touch sensing respectively.
The touch overlay described in the U.S. Pat. No. 4,686,332 suffers from a number of disadvantages. A finger touch on the overlay is sensed by a small change in the capacitance of the individual conductors. The capacitance can be measured with a variable frequency oscillator which connects the individual conductors to the period controlling capacitor of the oscillator. When no finger is touching the overlay, the oscillator runs at a frequency determined by the ambient capacitance of the overlay cables and electronic circuit capacitance. By studying the touch input system and its applications, certain goals, such as minimizing the ambient overlay capacitance, and thereby maximizing the percent change in capacitance when the overlay is touched, can be appreciated. Lower ambient capacitance is desired for applications that require greater speed and accuracy in finger touch operation. With only 2 mils separation between horizontal and vertical conductors, the ambient capacitance to ground is not minimized because the conductor that is being sensed passes relatively close to all the conductors in the other layer which were grounded during that measurement.
Another problem with the overlay structure described in the '332 patent is that the transparent conductors in the overlay are susceptible to microcracks, particularly in applications with a high stylus use. The transparent conductors described in the '332 patent are preferably comprised of indium tin oxide (ITO) because of its transparent and conductive properties. However, as ITO is a ceramic material, it is relatively brittle. After repeated pressure by the tip of a stylus, microcracks can develop in the overlay conductors causing eventual failure of the touch overlay. This problem is aggravated by formation of the upper conductors on the lower surface of the upper substrate, i.e. oriented down, toward the display surface in the same manner as those in a mechanical deformation membrane. When an ITO conductor faces toward the display, it is subject to predominantly tensile forces and is stretched at the point of contact of the stylus on the overlay surface. ITO is very susceptible to cracking problems when subjected to tensile forces. On the other hand, ITO is known to be relatively immune to compression forces. If the upper conductors could be oriented upwards away from the display, they would be subject to compressive forces, and therefore, less likely to crack.